The demand for the quick, economic and reproducible production of patterns on the nano or micro scale is one of the primary motivations in the development of nanotechnology.
The so-called nanoimprint, a recent lithography technology for fabricating nanoscale patterns in the field of nanoelectronics, photonics and biotechnology is a cost-effective method. The basic idea is the replication of patterns applied on, for example etched into, a stamp, by pressing this patterned stamp surface into a curable, flowable low-viscosity resist on a substrate surface, i.e. by means of simultaneous embossing of the resist and applying (imprinting) the substrate. After filling all cavities of the patterned stamp surface with the resist, the lacquer layer is cured by means of heat and/or ultraviolet light. In the final step, the stamp is removed and a three-dimensional replication of the pattern remains in the resist coating on the substrate. The nanopattern can then be transferred into a substrate by further etching. The method is described, for example, in 2007059497 A1.
Different methods are known for applying the resist coating which is yet to be embossed onto the stamp surface or onto the substrate. In the above-mentioned US 2007059497 A1, this is accomplished by the resist being applied onto the patterned stamp surface by means of rotation coating. U.S. Pat. No. 6,334,960 B1 describes the dispensing of resist lacquers onto the substrate in order then to emboss the resist surface on the substrate by means of the patterned stamp surface. The dispensing is comparatively time-consuming and the resist drop boundaries can lead to defects in the embossed resist after curing.
A roll-shaped stamp with a patterned roll surface, which rolls on the substrate provided with a resist coating in order to emboss the resist coating is known from U.S. Pat. No. 5,425,848 A1. The resist coating is cured almost simultaneously by UV irradiation in the area of the nip.
A method is known from WO 02/03142 A2 in which a polydimethylsiloxane stamp is wetted with an organic liquid in order to form self assembled monolayers (=SAMs) on its surface, and wherein the stamp with the SAMs is pressed onto the substrate in order to transfer the SAMs onto the substrate.
In the nanoimprint of the prior art, one is faced with two problems or aims, namely on the one hand, to achieve a uniform homogeneous filling of the patterned stamp surface, on the other hand, the residual resist thickness is supposed to be applied as thinly as possible, as will be explained below, in order not to delay of affect the subsequent etching treatment of the substrate.
These aims compete against each other; because the uniform filling is usually achieved with an excess amount of resist, there always remains an unpatterned residual resist thickness (thickness of the cured resist under the pattern), which may additionally have a layer thickness inhomogeneity. If the pattern of the stamp surface is not periodic but irregular, this inhomogeneity can become even greater because, when filling larger volumes of the pattern, more resist is consumed for filling than in smaller pattern volumes, so that lacquer deficiency and filling defects may arise. Though this homogeneity can be reduced, even if higher-viscosity lacquers are used, by using lacquer with a higher lacquer density, i.e. a thicker lacquer layer, for filling the pattern. However, the use of such a lacquer makes the subsequent etching process for transferring the pattern into the substrate as it is usually employed in the semiconductor industry more difficult.
Though a higher pressure during embossing can be used in order to reduce the inhomogeneity, this, however, does not work in the case of flexible stamps and moreover only provides for a minimal reduction of the residual resist thickness as a rule.
In order to increase homogeneity while simultaneously minimizing the residual resist thickness, it is known to carry out the coating process adapted to the pattern by locally adapting the density and size of the drops to the pattern during drop dispensing. This requires a precise analysis of the pattern and therefore is comparatively complex and time-consuming. The instrumentation setup is comparatively complicated and the above-mentioned problem of defects at the resist drop boundaries remains.
Therefore, the invention is based on the object of providing a method for applying a patterned coating of resist onto a substrate surface with a resist coating that is improved over the prior art in that it is more homogeneous and has a lower residual resist thickness, and that the method can be carried out easily, quickly and cost-effectively.